Try using System Restore to a point before the mschine slowed.
Also, try using the Task Manager to see if the cpu is being used more than just a few percent with no programs running.
Try turning off "Indexing service" if it is running.
There are many, many things that can cause this behavior such as a misbehaving usb device. Microsoft Knowledge Base is pretty good at helping to isolate these problems.
Worse case: Reformat HD and reinstall XP. Those data backups will come in handy.